I think the weapons crit individually so its better to have the guaranteed crit that comes with rogue.
Or Rage will guarantee crit.
Mortal Blow is the strongest damage you can do (500% I think) but I prefer dual-wield Warrior. Too much sneaking and wandering around with a Rogue - doesn't appeal to me. Vault also doesn't always put you in backstab position which negates the crit advantage.
It is a matter of taste I guess - whatever you prefer. I also always go for dual wield warrior over a single weapon as it is just cooler - according to all TV shows I've seen about Vikings and Pirates lately anyway.